85 



105.0 



0? 



0? 



1.17 



89 



80.4? 



99.6" 











1.01 



98? 



80. 



90. 











1.00 



90 



92.2 



97.8 



.044 



+ 7.2? 



1.17 



89 



90. 



90. 



.065 



+ 7.4 ? 



1.12 



87 



87.7 



82.3 



.07 



+ 7.7 ? 



1.00 



70 



1836.] of the Wet-bulb Hygrometer. 403 



fluctuations of the dry thermometer being so considerable for a nearly 

 stationary temperature of evaporation, it was somewhat difficult to 

 determine the exact terms of coincidence ; but the above are selected 

 as the best from a great many readings recorded in my note book. 



In a second experiment with air containing l^thsof aq. ten. at 

 94°.3 (= r g(j thsat 170°) the dry thermometer became stationary at 

 170°, with the wet at 87°.7. 



In a third trial, aq. ten .65 at 94.6 (= *hSs at 180) the stationary 

 points were 180, and 90. 



In a fourth, dew-pt. 74.3 (aq. ten. = 4.4 at 190) the same points 

 were 190 and 92.2— Bar. 29.50. 



Barometer Thermora. iu Wet-bulb Observed Cen. aq. Corr. Aqueous Quotient 

 curt, of air. Thermoin. depression, tens, fordo, tens, at V D -i-f 

 ooo **' 



29.55 190 



180 



170 

 29.50 190 



180 



170 



Observing the very rapid increase of the evaporating depression 

 with the rise of the temperature, I perceived that I might safely carry 

 my experiment to much higher limits than the boiling point of water. 

 I accordingly next passed the current of dry air through a porcelain 

 tube maintained at a bright orange heat in a Black's furnace (fig. 3, 

 PI. XXI.) At the further end of the tube a lateral hole was perforated 

 to admit the bulb of the thermometer, coated with two-fold muslin 

 that it might hold a larger supply of moisture. It was necessary to 

 watch the experiment carefully, as, the moment the water was removed, 

 a sudden rise took place, which would have otherwise broken the thermo- 

 meter, while the cloth and cork were instantly charred with the heat. The 

 actual temperature of the dry current was then estimated in the following 

 manner : a thermometer, with the tip of its stem left open, was held 

 mid- tube in the position previously occupied by the wet-bulb. In a 

 few minutes the mercury boiled off, shewing that the temperature 

 somewhat exceeded 656°. A very thin slip of tin was instantly fused : 

 one of lead was then held within the tube, but it required to be 

 passed a little in advance of the position of the wet-bulb ere it 

 melted : — we may therefore assume the heat of the dry air to have 

 been under 700°. Two experiments agreed precisely in giving the 

 temperature of evaporation 145°. "With a very rapid current the wet- 

 bulb thermometer fell to 144°, but probably the air had not then time 

 to get thoroughly heated in traversing the furnace. 

 3 f 2 



